Methods and apparatus for examination and measurement by means of nuclear magnetic resonance phenomena

ABSTRACT

In nuclear magnetic resonance apparatus a sample under test is subjected to a first unidirectional magnetic field and to a second alternating magnetic field perpendicular to said first magnetic field, and the field strength of said first magnetic field and/or the alternation frequency of said second magnetic field is modulated in such manner that the sum of two successive time intervals between detected resonance pulses from the sample is not constant.

o Muted States Patent [151 3,638,104 Wright [451 Jan. 25, 1972 METHODSAND APPARATUS EUR References Cited 1 1 EXAMENATHQN AND MEASUREMEN'EUNITED STATES PATENTS BY MEANS OF NUCLEAR MAGNETIC 2 894 I99 7/1959 h314/0 5 9 1rc ner REONANCE PHENOMENA 3,289,072 ll/l966 Schuster [72]inventor: Reginald Graham Wright, Newport Pagnell, England PrimaryExaminer-Michael .I. Lynch Atlorne vYoung & Thompson [73] AssigneezNewport Instruments Limited, Newport,

Pagnell, England 57 ABSTRACT [22] Filed: June 1969 In nuclear magneticresonance apparatus a sample under test [2]] App]. No; 829,679 issubjected to a first unidirectional magnetic field and to a secondalternating magnetic field perpendicular to said first magnetic field,and the field strength of said first magnetic g" Application Pflomy Datafield and/or the alternation frequency of said second magnetic June 71968 Great Britain I 27 200/68 field is modulated in such manner thatthe sum of two successive time intervals between detected resonancepulses from 52 us. 01 ..324/0.s the Sample is [51] int. Cl. (20in 27/7813 i 8D [58] Field of Search ..324/0.5 C

I 5 Half 76 Ii 22 73 23 72 n I l ru .1 ,1 .1 .1 -r 14 I4 EEfi/ERll'fO/ZGAE/VERA 7011 H I H 2 METHODS AND APPARATUS FOR EXAMINATION ANDMEASUREMENT BY MEANS OF NUCLEAR MAGNETIC RESONANCE PHENOMENA Thisinvention relates to methods of and apparatus for examination andmeasurement by means of nuclear magnetic resonance phenomena, and ismore particularly concerned with the examination and measurement ofsamples which have long relaxation times and which saturate easily.

The use of the nuclear magnetic resonance phenomenon for examinationpurposes is well known. Broadly, a sample of a substance under test issubjected to a first unidirectional magnetic field and to a secondalternating magnetic field perpendicular to the first field. At aparticular alteration frequency of the alternating field which isrelated to the strength of the unidirectional field, power is absorbedfrom the alternating field, and from this power absorption an electricalsignal may be derived which is indicative of the condition of resonance.

In most practical nuclear magnetic resonance circuit arrangementsprovision is made to modulate or sweep either the alteration frequencyof the alternating magnetic field, or, more usually, the field strengthof the unidirectional field, in order to embrace the respectiveresonance conditions for a number of different materials and therebyallow determination of the identity of the particular sample of materialunder examination. This modulation or sweeping of the unidirectionalfield or of the alternation frequency of the alternating magnetic fieldresults in the production of a resonance signal having the form of abell-shaped pulse each time the modulation causes the magnetic field topass through the particular value establishing the resonance conditionfor the sample.

In United Kingdom Pat. No. 1,125,482 there is described nuclear magneticresonance apparatus comprising a magnet providing a unidirectionalmagnetic field, a modulation coil for superimposing a small variation onthe steady unidirectional field, and a resonance detector circuitconnected to a coil surrounding the sample and through which ahigh-frequency oscillatory magnetic field can be applied to the sample.

In a typical measurement with this known form of apparatus, a cyclicallyvarying current is applied to the modulation coil for cyclically varyingthe intensity of the unidirectional field. The frequency of the detectoroscillator circuit is then adjusted until a resonance peak is observedeach time the modulation causes the resultant magnetic field to passthrough the value of the unidirectional field.

With samples having relaxation times much shorter than the period of themodulating waveform, a single pulse is observed each time the resultantmagnetic field passes through the value for resonance. With sampleshaving relatively long relaxation times, however, each pulse may befollowed by an oscillatory decaying waveform, which is a well-knownphenomenon described as ringing. When the relaxation time of the samplebecomes of the same order of magnitude or longer than the period of themodulating waveform, the ringing may not decay to zero before the nextresonance pulse occurs. This ringing effect may therefore build up to ahigh level and interfere seriously with the measurement of the resonancesignals. External interference causing changes in the unidirectionalmagnetic field or in the oscillator frequency alters the ringingwaveform and may cause large variations in the resonance pulses.

Previous techniques for avoiding these troublesome effects require theuse of very low levels of oscillator power, with the consequentdisadvantage of a low signal-to-noise ratio for the detected resonancepulse signals.

It is the primary object of the present invention to suppress thisringing by modifying the form of modulation applied to the nuclearmagnetic resonance apparatus.

This is achieved in accordance with the present invention by subjectinga sample to a first unidirectional magnetic field and to a secondalternating magnetic field perpendicular to said first magnetic field,and modulating the field strength of said first magnetic field and/orthe alternation frequency of said second magnetic field whereby anoutput signal indicative of resonance is obtained each time theresultant magnetic field passes through the resonance condition for thesample, the modulation being such that the sum of two successive timeintervals between output signals from a particular sample is notconstant.

In a preferred form of the invention, the field strength of theunidirectional magnetic field is modulated by a primary cyclicmodulation and by a secondary modulation superimposed on the primarymodulation. For example, the primary modulation may be represented by atriangular waveform and the secondary modulation by a sinusoidalwaveform.

The invention may be applied to situations where it is required tosuppress the ringing effect or the buildup of ringing; where it isrequired to reduce the level of saturation of a sample; where it isrequired to improve the stability of a resonance signal from a sampleexhibiting saturation in the presence of interference causingdisturbances to the magnetic field or to the frequency of the resonanceoscillator or to the frequencies used for measurement; where it isrequired to indicate or measure the presence of long relaxation times orof saturation, by altering or switching off the secondary modulation andobserving the effect on the resonance signals; where it is required todistinguish between the signals from different components of a samplehaving different relaxation times; or where it is required to obtain animproved signal-to-noise ratio for the detected resonance pulses.

In order that the invention may be fully understood, a number ofembodiments in accordance with the invention will now be described byway of example and with reference to the accompanying drawings, inwhich:

FIG. 1 is a block schematic diagram of gyromagnetic resonance apparatusin accordance with the invention and including first and secondgenerators of primary and secondary modulation respectively;

FIGS. 2(a), 2(b) and 2(0) illustrate the known method of modulating thefield by a single cyclically varying modulation;

FIGS. 2(d) and 2(e) illustrate the principle of the present inventionwhich uses a modified form of modulation; and,

FIGS. 3 to 8 are schematic illustrations showing alternative ways ofachieving a modified form of modulation in accordance with theinvention.

Reference is first made to FIGS. 1 and 2 of the drawings. Thearrangement shown in FIG. 1 comprises a magnet structure 10 illustratedby opposing pole pieces 11 and 12 for providing a homogeneousundirectional magnetic field within which a material sample 13 isplaced. The pole pieces 11 and 12 are surrounded by magnet windings 14for energization by a suitable direct current source, shown symbolicallyas a battery 15. A variable series resistor 16 in the supply leads tothe windings l4 permits adjustment of the field intensity provided bythe windings.

A further winding 17 surrounds the sample 13 and is connected to aself-oscillating nuclear magnetic resonance detector 18, whereby thewinding 17 is supplied with a highfrequency oscillation in order toprovide a high-frequency magnetic field lying at right angles to theunidirectional magnetic field and which is necessary for theestablishment of nuclear magnetic resonance within the material sample13. The winding 17 forms part of the resonant tank circuit of thedetector 18 and a variable capacitor 19 is connected across the winding17 to allow adjustment of the oscillation frequency of the detector 18.

The derived resonance pulse signals at the output of the detector 18 arefed to an amplifier 20, and the output therefrom is supplied to arecorder 21 of suitable form.

Adjacent to the pole pieces 11 and 12 are provided sweep coil windings22 and 23 respectively (hereinafter referred to as modulation windings).The modulation winding 22 is connected to a first current waveformgenerator 24 and the modulation winding 23 is connected to a secondcurrent waveform generator 25.

In FIGS. 1 and 2 and in the following description the value of theunidirectional magnetic field provided by the pole pieces 11 and 12 isreferred to as Hm; the modulation field provided by the winding 22 andgenerator 24 is referred to as H1; and the modulation field provided bythe winding 23 and generator is referred to as H2.

In the known systems, such as described in United Kingdom Pat. No.l,l25,482, only one generator and modulation winding are used and thegenerator supplies a simple cyclically varying current to the winding.In FIG. 2(a) which represents this known manner of modulating thestrength of the unidirectional field, the unidirectional field isindicated by the horizontal broken line Hm and is plotted as a functionof time, and the resultant modulated field Hm+H1 is indicated as aregular triangular waveform. In operation, the cyclically varyingcurrent from the generator, such as 24, to the modulation winding causesthe otherwise steady value of the unidirectional field to be increasedand decreased regularly once in each cycle period of the modulationwaveform. At particular values of the unidirectional field related tothe operating frequency of the high-frequency oscillation supplied bythe detector circuit 18 to the winding 17, nuclear resonance occurwithin the sample 13. The frequency of the oscillator 18 may be adjustedso that the resonance peaks will be observed each time the resultantmodulated field passes through the value Hm, as shown in FIG. 2(b).Should the oscillator frequency be slightly higher or lower, resonancewill occur at slightly lower or higher field values respectively, asshown, for example, in FIG. 2(0).

It will be noted however that with this known method of modulating themagnetic field using the one generator 24 and a single modulationwinding 22 and employing the cyclic triangular modulation waveformshown, that the time intervals between the resonance peaks are regularand the sum of two successive time intervals between peaks is equal tothe period of the modulation waveform H1. In other words, the timeinterval (t,+r between the peaks 1 and 3 or the time interval (t,+tbetween the peaks 2 and 4, for example, of FIG. 2(b) is constant andequal to the period T of the waveform H1. Similarly, in FIG. 2(0) thetime interval (t,+t between the peaks 1 and 3 or the time interval (t +tbetween the peaks 2 and 4, for example, is likewise constant and equalto the period T of the modulating waveform H1.

. In accordance with the present invention, this known modulationarrangement illustrated by FIGS. 2(a), 2(b) and 2(c) using a singlemodulation generator is modified, as shown in FIG. 1, by the addition ofthe second generator 25 and its as- Sociated winding 23. In a preferredform of the invention, the generator 24 superimposes a triangularwaveform H1 of about 3 gauss peak-to-peak amplitude at 33 Hz. onto thesteady magnetic field Hm and the second generator 25 superimposes atriangular waveform H2 of for example between 0.02 and 0.5 gausspeak-to-peak amplitude at a frequency of about 2 Hz. onto the modulatedfield, giving a resultant field variation (Hm+Hl+H2) as shown in FIG.2(d), only a part of the waveform H2 being shown in FIG. 2(d) forclarity. If the frequency of the detector oscillator 18 is adjusted sothat the resonance peaks occur at a field value Hm, resonance peaks willbe observed at the time instants shown in FIG. 2(e). It will be notedthat the time intervals between successive peaks 1, 2, 3, 4 etc., are nolonger regular as in FIGS. 2(b) and 2(a), but vary according to thevalue of the secondary modulation waveform H2. In other words, the timeinterval (r,+r no longer equals the time interval (t +t and so on. Bythe use of this secondary modulation H2 the intense ringing" which maybuild up with samples having long relaxation times is suppressed.

Physically, the action of the secondary modulation used with the presentinvention may be described as preventing a phase coherence from beingestablished between the precessing nuclei and the oscillator 18. Thismay reduce the level of saturation which can be built up by the nucleior atomic particles, and this phase incoherence may induce relaxation,thus further lowering the level of saturation.

Although the embodiment described above uses secondary modulation H2having a triangular waveform, a sinusoidal waveform may alternatively beused.

It should further be realized that either or both of the primary andsecondary modulations may have waveforms other than triangular orsinusoidal, and may have frequencies and amplitudes differing from thespecific values given above which are by way of illustration only. Inparticular, the secondary modulation provided by the generator 25 andthe winding 23 may have a random waveform rather than a cyclicallyvarying waveform.

The present invention has so far been described in terms of two separategenerating systems providing for example two triangular modulations orone triangular and one sinusoidal modulation, but it will be apparentthat the broad principle of the present invention, i.e., modulating themagnetic field in such manner that the resonance pulses occurirregularly as shown in FIG. 2(e), can be achieved in many other ways. Anumber of such alternative modulating arrangements will now be describedwith reference to FIGS. 3 to 8, bearing in mind that in each case themodulation is such as to produce resonance pulses where the sum of twosuccessive time intervals between pulses is not a constant value. Inthese diagrammatic representations certain of the elements shown in FIG.1 have been omitted for clarity, but in practice the basic system ofFIG. 1 would be used.

As shown in FIG. 3, the primary and secondary modulations H1 and H2 maybe combined as a single modulation H12 of complex waveform provided by acomplex waveform generator 30 having an associated modulation winding31.,

In the embodiment shown in FIG. 4, the apparatus includes only thesingle generator 24 of modulation HI and its modulation winding 22, andthe secondary modulation of the present invention is provided by afrequency control circuit 32 which is connected to the generator 24 andwhich varies the frequency of the modulation waveform H1. This will varythe time intervals between the resonance peaks. Alternatively, thesecondary modulation may be provided by varying the shape of thewaveform H1. In this special case the time intervals between resonancepeaks may be regular but the angle through which the nuclei haveprocessed may be made to vary.

The modulations have heretofore been described as being applied to themagnetic field by coil means, but modulation of the magnetic field maybe achieved in other ways. As shown in FIG. 5, four magnets 33, 34, 35and 36 are positioned around the gap between the pole pieces 11 and 12where the sample 13 is mounted. These magnets 33 to 36 are mounted forreciprocating movement relative to the sample and to the pole pieces 11and 12 and by a suitable movement of the magnets the primary andsecondary modulation of the field may be achieved. In general, thedesired modulation may be achieved by the variation or movement orrotation of magnetic material or a magnet or magnets in a suitableposition relative to the unidirectional magnetic field.

The supplementary modulation efi'ect of the present invention may alsobe achieved by using a magnetic field which varies with position insteadof with time, and by moving the sample relative to this field in such away that the nuclei or atomic particles in the sample experience a fieldvarying with time. The sample movement may be rotational ortranslational or a combination of these, and FIG. 6 schematicallyillustrates a translational sample movement.

The modulation effects described above have been achieved by varying themagnetic field applied to the sample, but it will be realized by thoseskilled in the art that since the effects of small variations ofmagnetic field may be reproduced equally by equivalent small variationsof frequency, the same result can be achieved if one or other or both ofthe primary and secondary modulations are modulations of the frequencyof the oscillator detector 18.

FIG. 7 illustrates an embodiment in which a single modulation generator24 is used with its associated modulation winding 22, and associatedwith the oscillator 18 is a frequency controller 37 which provides asingle-mode modulation of the oscillator frequency, whereby thegenerator 24 provides the primary modulation and the frequencycontroller 37 provides the secondary modulation required by theinvention.

Alternatively, as shown in FIG. 8, the generator 24 and winding 22 canbe omitted altogether and both the primary and secondary modulationsrequired by the invention are achieved by the use of a compoundfrequency controller 38 connected to the oscillator 18 and arranged tomodulate the frequency of the oscillation in first and second modes.

The invention has been particularly described in connection with agyromagnetic resonance oscillator 18, but other forms of magneticresonance detector may equally well be used.

I claim:

1. Magnetic resonance apparatus comprising resonance detecting meanshaving a sensing coil arranged to surround a test material sample andincluding means to induce oscillation in the sensing coil to provide analternating magnetic field, means for establishing a unidirectionalmagnetic field perpendicular to said alternating magnetic field in theregion of said sensing coil, and modulation means effective to sweepsaid unidirectional field causing the resultant magnetic field to passthrough the resonance condition for the sample and hence generate anoutput signal indicative of resonance at successive time instants whichare spaced at intervals such that the sum of two successive timeintervals is not constant.

2. Apparatus as claimed in claim 1, in which said modulation meansmodulates the intensity of said unidirectional field and includesgenerator means providing at least one modulation component having apeak-to-peak amplitude many times greater than the resonance line width.

3. Apparatus as claimed in claim 1, in which said modulation meanscomprises first and second windings positioned adjacent to said sensingcoil in the unidirectional field, and first and second waveformgenerators respectively connected to said first and second windings,said first waveform generator being arranged to energize said firstwinding to superimpose a primary modulation of a first amplitude on saidunidirectional field and said second waveform generator being arrangedto energize said second winding to superimpose a secondary modulation ofa second amplitude on said unidirectional field.

4. Apparatus as claimed in claim 3, wherein said first generator andsaid second generator both provide cyclic modulation waveforms.

5. Apparatus as claimed in claim 3, wherein said first generatorprovides a cyclic modulation waveform and said second generator providesa random modulation waveform.

6. Apparatus as claimed in claim 1, wherein said modulating meanscomprises a winding located adjacent to the sensing coil in theunidirectional magnetic field and a generator connected to said windingand arranged to supply a single modulation of complex noncyclicwaveform.

7. Apparatus as claimed in claim 3, wherein said first generator isarranged to provide a triangular waveform and said second generator isarranged to provide a sinusoidal waveform.

8. Apparatus as claimed in claim 1, wherein said modulating meanscomprises'a winding located adjacent to the sensing coil in theunidirectional magnetic field, a generator connected to said winding andarranged to supply a cyclic modulation waveform thereto, and meansconnected to said generator and arranged to vary the frequency of thecyclic waveform produced thereby.

9. Apparatus as claimed in claiml wherein said modulating meanscomprises at least one element of magnetic material positioned adjacentto said means establishing a magnetic field and arranged to be movablerelative thereto to produce the modulation of the field strength of theunidirectional magnetic field.

10. Magnetic resonance apparatus comprising resonance detecting meanshaving a sensing coil arranged to surround a test material sample andincluding means to induce oscillation in the sensing coil to provide analternating magnetic field,

means for establishing a unidirectional magnetic field perpendicular tosaid alternating magnetic field in the region of said sensing coil, andmodulation means effective to sweep the frequency of the alternatingmagnetic field causing the resultant magnetic field to pass through theresonance condition for the sample and hence generate an output signalindicative of resonance at successive time instants which are spaced atintervals such that the sum of two successive time intervals is notconstant.

11. Apparatus as claimed in claim 10, in which said sensing coil isconnected as part of the resonant tank circuit of a selfoscillatingnuclear magnetic resonance detector, and said modulation means comprisesmeans connected to the detector and operative to modulate the frequencyof the detector in first and second modes.

12. Magnetic resonance apparatus comprising resonance detecting meanshaving a sensing coil arranged to surround a test material sample andincluding means to induce oscillation in the sensing coil to provide analternating magnetic field, means for establishing a unidirectionalmagnetic field perpendicular to said alternating magnetic field in theregion of said sensing coil, and first and second modulation meansrespectively effective to sweep the unidirectional field and thefrequency of the alternating magnetic field causing the resultantmagnetic field to pass through the resonance condition for the sampleand hence generate an output signal indicative of resonance atsuccessive time instants which are spaced at intervals such that the sumof two successive time intervals is not constant.

13, Apparatus as claimed in claim 12, in which said first modulationmeans modulates the intensity of said unidirectional field and includesgenerator means providing a modulation having a peak-to-peak amplitudemany times greater than the resonance line width.

1. Magnetic resonance apparatus comprising resonance detecting meanshaving a sensing coil arranged to surround a test material sample andincluding means to induce oscillation in the sensing coil to provide analternating magnetic field, means for establishing a unidirectionalmagnetic field perpendicular to said alternating magnetic field in theregion of said sensing coil, and modulation means effective to sweepsaid unidirectional field causing the resultant magnetic field to passthrough the resonance condition for the sample and hence generate anoutput signal indicative of resonance at successive time instants whichare spaced at intervals such that the sum of two successive timeintervals is not constant.
 2. Apparatus as claimed in claim 1, in whichsaid modulation means modulates the intensity of said unidirectionalfield and includes generator means providing at least one modulationcomponent having a peak-to-peak amplitude many times greater than theresonance line width.
 3. Apparatus as claimed in claim 1, in which saidmodulation means comprises first and second windings positioned adjacentto said sensing coil in the unidirectional field, and first and secOndwaveform generators respectively connected to said first and secondwindings, said first waveform generator being arranged to energize saidfirst winding to superimpose a primary modulation of a first amplitudeon said unidirectional field and said second waveform generator beingarranged to energize said second winding to superimpose a secondarymodulation of a second amplitude on said unidirectional field. 4.Apparatus as claimed in claim 3, wherein said first generator and saidsecond generator both provide cyclic modulation waveforms.
 5. Apparatusas claimed in claim 3, wherein said first generator provides a cyclicmodulation waveform and said second generator provides a randommodulation waveform.
 6. Apparatus as claimed in claim 1, wherein saidmodulating means comprises a winding located adjacent to the sensingcoil in the unidirectional magnetic field and a generator connected tosaid winding and arranged to supply a single modulation of complexnoncyclic waveform.
 7. Apparatus as claimed in claim 3, wherein saidfirst generator is arranged to provide a triangular waveform and saidsecond generator is arranged to provide a sinusoidal waveform. 8.Apparatus as claimed in claim 1, wherein said modulating means comprisesa winding located adjacent to the sensing coil in the unidirectionalmagnetic field, a generator connected to said winding and arranged tosupply a cyclic modulation waveform thereto, and means connected to saidgenerator and arranged to vary the frequency of the cyclic waveformproduced thereby.
 9. Apparatus as claimed in claim 1, wherein saidmodulating means comprises at least one element of magnetic materialpositioned adjacent to said means establishing a magnetic field andarranged to be movable relative thereto to produce the modulation of thefield strength of the unidirectional magnetic field.
 10. Magneticresonance apparatus comprising resonance detecting means having asensing coil arranged to surround a test material sample and includingmeans to induce oscillation in the sensing coil to provide analternating magnetic field, means for establishing a unidirectionalmagnetic field perpendicular to said alternating magnetic field in theregion of said sensing coil, and modulation means effective to sweep thefrequency of the alternating magnetic field causing the resultantmagnetic field to pass through the resonance condition for the sampleand hence generate an output signal indicative of resonance atsuccessive time instants which are spaced at intervals such that the sumof two successive time intervals is not constant.
 11. Apparatus asclaimed in claim 10, in which said sensing coil is connected as part ofthe resonant tank circuit of a self-oscillating nuclear magneticresonance detector, and said modulation means comprises means connectedto the detector and operative to modulate the frequency of the detectorin first and second modes.
 12. Magnetic resonance apparatus comprisingresonance detecting means having a sensing coil arranged to surround atest material sample and including means to induce oscillation in thesensing coil to provide an alternating magnetic field, means forestablishing a unidirectional magnetic field perpendicular to saidalternating magnetic field in the region of said sensing coil, and firstand second modulation means respectively effective to sweep theunidirectional field and the frequency of the alternating magnetic fieldcausing the resultant magnetic field to pass through the resonancecondition for the sample and hence generate an output signal indicativeof resonance at successive time instants which are spaced at intervalssuch that the sum of two successive time intervals is not constant. 13.Apparatus as claimed in claim 12, in which said first modulation meansmodulates the intensity of said unidirectional field and includesgenerator means providing a modulation having a peak-to-peak amplitudemany times greater than the resonance line width.